1. Field of the Invention
The present invention relates to a method for monitoring a rotation of a compressor wheel of a turbocharger.
2. Description of the Related Art
In passenger car applications as well as in commercial motor vehicle applications, exhaust-gas turbochargers are increasingly used in order to increase the efficiency of the engine output. Today, already approximately 30% of commercial motor vehicles have an exhaust-gas turbocharger, tendency rising. As is known, such applications of exhaust-gas turbochargers are used to secure and increase the reliability as well as for the purpose of an improved utilization of the system limits. For monitoring the exhaust-gas turbochargers, sensors are normally used, which monitor the rotation of the rotating turbine wheels or compressor wheels, for example. In twin-turbo systems or sequential turbocharging, the sensors are also required for regulating the exhaust system and/or for coordinating the turbochargers.
Numerous sensors are known from the related art, which detect at least one rotational property of rotating elements such as the compressor wheel or the compressor wheel of an exhaust-gas turbocharger. Rotational properties are generally to be understood as properties that at least partially describe the rotation of the rotating element. For instance, these may be angular speeds, rotational speeds, angular accelerations, angles of rotation, angular positions or other properties that could characterize a continuous or discontinuous, uniform or nonuniform rotation or turning of the rotating element. Examples of such sensors are described in Konrad Reif: Sensors in a motor vehicle [Sensoren im Kraftfahrzeug], 1st edition 2010, on pages 63 through 73 and 120 through 129. A particular point of concentration of the present invention, to which the present invention is not limited, however, is a detection of rotational speed.
To detect a rotational speed of a turbocharger, for example, it is possible to ascertain the rotational speed of the compressor wheel of the turbocharger via a detection of the individual moving compressor blades. Sensors may be used for this purpose, for example, which are based on a passive-inductive measuring principle. According to this measuring principle, the compressor blades following a rotary motion are guided through a stationary primary magnetic field, which causes in a known manner eddy currents in the compressor blades, which are preferably made of light metal, which eddy currents in turn generate a separate so-called secondary magnetic field. In the area of the sensor, which normally has a coil as sensor element, the secondary magnetic field produces an inductive current that is periodic with the rotation of the compressor blades and changes over time, and which may be used by the sensor as an output signal for example for computing the rotational speed of the compressor wheel. The alternating current induced in the coil of the sensor head normally depends for example on the speed of the passing compressor blades, the strength of the material-specific eddy field of the compressor blades, the distance between the individual compressor blades as well as the distance between the compressor blades and the sensor head. Particularly in the area of commercial vehicles, high reliability requirements are placed on a function of the vehicle, because a failure, particularly due to a breakdown of an exhaust-gas turbocharger, is usually associated with a time-consuming repair and an equally time-consuming loss of use of the vehicle. In the usual commercial use of the vehicles, the latter has negative economic consequences.
Consequently, it is not only necessary to monitor and ensure a current functioning of the turbocharger, but furthermore a prediction of the probability of failure as well as an early warning based on such prediction of a failure of the turbocharger are required.